Both air-cooled heat exchangers and thermal bushings are known technology which have been utilized as separate items in the sealing industry for many years. However, to the best of my knowledge, the combined use of an air-cooled heat exchanger and a thermal bushing in an environmental control system for a mechanical seal assembly has never been attempted. Thus, this invention is concerned with an improved environmental control system, particularly a bypass flush system, which is believed to eliminate a long standing problem in the sealing industry.
Mechanical seal assemblies have long been utilized with fluid handling apparatus (such as pumps, autoclaves and the like) which are used in conjunction with high temperature and/or corrosive fluids, particularly in the chemical and petrochemical industries. In such utilizations, the mechanical seal assembly is normally provided with an environmental control system which supplies a fluid into the seal cavity to control the temperature thereof. This control system generally involves a bypass line which bleeds off of the main flow line at a location downstream of the pumping apparatus, which bypass line flows through an appropriate heat exchanger and then connects to the seal cavity. To control the rate at which the flushing fluid flows through the bypass line, it has been conventional to provide the bypass line with a flow valve located either upstream or downstream of the heat exchanger. This flow control valve normally comprises an orifice or an adjustable needle valve. While the required flow rate for the flushing fluid is preferably small, particularly since the flushing fluid is normally the same fluid being handled by the pumping apparatus, nevertheless experience indicates that proper operation of the bypass system requires that the flow rate be maintained rather large in order to ensure dependable operation thereof. More specifically, when handling high-temperature dirty fluids such as Dowtherm, Therminol, crude oil and the like, these fluids frequently carbonize or coke-out, thereby leaving debris in the system which deposits in the orifice or on the needle valve, thereby plugging the valve and rendering it useless. These deposits thus effectively close off the bypass system and prevent circulation of the flushing fluid through the seal cavity. To avoid this long standing problem, it has been conventional to utilize a substantially larger valve opening. This results in the flow rate through the bypass system being many times larger than required for a specific application. For example, flow rates of flushing fluid up to ten gallons per minute are conventionally utilized, whereas in many instances a maximum flow rate of approximately one-eighth to two gallons per minute is sufficient provided that the system can be properly controlled and maintained operational over long periods of time.
Because of the necessity of having to utilize large flow rates, as noted above, this in turn results in a large increase in the required capacity of the heat exchanger used in conjunction with the bypass line. Thus, attempting to utilize an air-cooled heat exchanger under these circumstances becomes totally impractical since the required cooling area of an air-cooled heat exchanger, coupled with the high initial cost and extreme size and space requirements, thus completely outweigh any of the practical advantages achieved by utilizing an air-cooled heat exchanger. Thus, bypass systems have normally utilized a water-cooled heat exchanger since same requires a cooling surface area which is 20 to 30 times less than that of an equivalent air-cooled heat exchanger. However, as is well known, water-cooled heat exchangers themselves create additional problems with respect to the necessity of having to handle an additional fluid, namely water, and the required piping, seals, pumps and other apparatus required therefore. In most instances, however, the large size of an equivalent air-cooled heat exchanger thus dictate the utilization of a watercooled heat exchanger, even though same does possess disadvantages.
While bypass systems of the above-described type have possessed the above-described disadvantages, nevertheless these systems have been utilized for many years since no one has, prior to the present invention, arrived at a simplified system which overcomes these disadvantages and yet results in a dependable, simple and economical system.
In known fluid handling apparatus employing mechanical seal assemblies, various types of seals have been provided between the mechanical seal cavity and the fluid handling apparatus. For example, attempts to prevent or at least minimize the communication between the pumping apparatus, such as the pumping chamber of a centrifugal pump, and the mechanical seal cavity have involved the use of fixed bushings, floating bushings, labyrinth seals and the like. While these seals have been primarily designed so as to attempt to prevent flow between the pumping chamber and the mechanical seal cavity, nevertheless minimizing or controlling the clearance within these seals is substantially impossible, so that a noncontrolled amount of fluid normally flows past these seals. These different seals, as mentioned above, have also created structure and wear problems which have made their usage less than desirable. These seals have accordingly not entered into the design of known bypass flush systems.
One of the problems associated with usage of a fixed bushing is the rubbing between the bushing and the rotatable shaft disposed therein, which results in excessive wear of the shaft. This thus has resulted in fixed bushings normally being constructed of soft metals or carbon, whereby the bushing is subject to the wear rather than the shaft. However, constructing the fixed bushing of carbon results in substantial differential thermal expansion between the shaft and the carbon bushing due to the dissimilar thermal expansion coefficients thereof, whereby the clearance between the bushing and the shaft undergoes substantial variation and hence excessive clearances must be provided between the shaft and the bushing at low temperatures. Because of these undesirable features of fixed bushings, their utilization has been extremely limited. Rather, floating bushings have been utilized more extensively since they radially float with the shaft and can thus be constructed of the same metal as the shaft to permit a uniform clearance to be maintained between the bushing and the shaft at all times, while at the same time not resulting in excessive shaft wear.
In addition to the use of both fixed and floating bushing seals for preventing leakage from the mechanical seal cavity, the sealing industry has also utilized so-called "Thermal bushings" for restricting flow from the mechanical seal cavity. Thermal bushings are a composite structure which attempts to utilize the best features of both the floating and fixed bushings.
The thermal bushing utilizes an inner sleeve of carbon or similar soft material. This inner carbon sleeve is shrunk into an outer steel sleeve which determines the expansion coefficient of the composite thermal bushing so that a substantially uniform clearance exists between the shaft and the thermal bushing at all times. At the same time, the inner carbon sleeve prevents undesirable shaft wear. Thermal bushings of this type are, however, normally utilized as a fixed bushing. Further, these known thermal bushings have, to the best of my knowledge, always been utilized as a seal for restricting the flow of a barrier fluid or the loss of a fluid from a vessel.
Accordingly, it is an object of the present invention to provide an improved environmental control system for use with a mechanical seal assembly, which control system is of the bypass flush type and is capable of handling hot, dirty liquids and employs a flow control device formed as a thermal bushing for permitting successful operation of the system while controlling flow of the flushing fluid at an extremely low rate. More specifically, it is an object of the invention to provide:
1. A bypass flush system for a mechanical seal assembly, as aforesaid, which permits the flow rate of the bypass flushing liquid, namely the hot dirty liquid, to be maintained at a rate several times smaller than previously felt feasible, such as a rate of two gallons per minute or less, and even less than one gallon per minute, without encountering sever maintenance problems due to plugging of the flow control device with dirt and deposits.
2. A system, as aforesaid, which employs a thermal bushing as the flow control device whereby an extremely low flow rate of flushing fluid can be continuously maintained therethrough, and wherein the clearance passage between the bushing and the shaft is maintained clean due to the vibration and rotation of the shaft.
3. A system, as aforesaid, which permits utilization of an air-cooled heat exchanger in association with the bypass line for cooling the flushing fluid flowing therethrough, which air-cooled heat exchanger can be of nominal size while effectively cooling the fluid to the desired extent due to the extremely small flow rate of the fluid.
4. A system, as aforesaid, which permits for more accurate and uniform control over the flow rate of the fluid even as the fluid and the overall system undergo substantial variations in temperature.
5. A system, as aforesaid, wherein the flow control device, namely the thermal bushing, is designed to compensate for temperature increases in the fluid so that, as the temperature increases, the clearance passage between the thermal bushing and the shaft progressively decreases even as the viscosity of the flushing fluid decreases, so that the flow rate of fluid through the system thus remains substantially constant at the desired low flow rate.
6. A system, as aforesaid, wherein the thermal bushing is positioned at the interface between the mechanical seal cavity and the pumping chamber so as to function as a barrier between these cavities, while at the same time permitting the flushing fluid to flow from the mechanical seal cavity past the thermal bushing directly into the pumping chamber to thereby resupply the fluid to the pumping system, thereby simplifying the overall environmental cooling system and the hardware and equipment required therefor.
7. A system, as aforesaid, which is operationally and structurally simple due to the utilization of a thermal bushing and an air-cooled heat exchanger for permitting the desired cooling of the flushing fluid while controlling the flow rate at an extremely low magnitude, which is of minimum size and minimum operational complexity by permitting the use of a simple and inexpensive aircooled heat exchanger in contrast to a water-cooled heat exchanger, which permits successful and dependable operation over long periods of time with the use of hot dirty liquids, which permits the utilization of a flow rate several times smaller than that previously thought feasible, and which permits the small flow rate of the fluid to remain substantially constant even though the overall system and apparatus undergoes substantial variation in temperature.
Other objects and purposes of the invention will be apparent to persons familiar with general systems of this type upon reading the following specification and inspecting the accompanying drawings.